Nonwoven fabrics are used in a wide variety of everyday applications, as for example, as components in absorbent products such as disposable diapers, adult incontinence pads and sanitary napkins, in medical applications such as surgical gowns, surgical drapes, sterilization wraps, and in numerous other applications such as disposable wipes, carpets and filtration media.
Nonwoven fabrics can be produced by a number of different methods. One such method is spinbonding, a significant and growing area that is becoming more prominent in the production of nonwovens. Various spinbonding techniques exist, but all include the basic steps of extruding continuous filaments, quenching the filaments, drawing or attenuating the filaments by a high velocity fluid, and collecting the filaments on a surface to form a web.
Spunbonded nonwoven webs exhibit several desirable characteristics. However, spunbonded nonwoven webs generally do not have a very high loft or volume and thus exhibit relatively low thickness for any given fabric weight. It would be advantageous, given the increasing use of nonwoven webs, especially as coverstock in disposable products, to produce a nonwoven web with increased loft. Coverstock produced from nonwoven webs with increased loft would exhibit desirable properties such as improved softness, greater fabric open area, increased dryness, improved strike-through and run-off properties, and the like. Further, for filtration applications of nonwoven webs, a web having increased loft would exhibit such desirable properties as increased filtration efficiencies and filtrant holding capacities.
There are various ways in which loft can be imparted to fabrics. For example, the fibers or yarns which form the fabrics may be crimped to thus impart bulk or loft to the resulting fabrics. Conventional methods for crimping yarns include mechanical processes such as stuffer box crimping, gear crimping, and false twist texturizing, for example. While such mechanical crimping processes can be used in some of the known methods for producing nonwoven fabrics, mechanical crimping processes are not applicable to the production of spunbonded nonwoven fabrics, due to the continuous nature of the spinbonding process.
Another approach to producing crimped filaments or yarns is through the use of structured matrix bicomponent or multicomponent fibers. Such fibers generally comprise two or more polymer components having differing compositions or thermal properties. When the fibers are processed, e.g. subjected to heat, they develop a crimp. However, this approach has its disadvantages as well. Typically bicomponent fibers are expensive and require specialized equipment and processing techniques.
The present invention is based on the discovery that a phenomenon known as melt-fracture can be used advantageously to produce nonwoven webs with increased loft. By appropriate control of the extrusion and drawing conditions, filaments are produced which exhibit a crimped structure. When applied to a continuous nonwoven fabric production method, such as spinbonding, a high-loft nonwoven fabric formed of the crimped melt-fractured filaments is produced in a continuous operation.
Melt-fracture is a form of melt flow instability in the extrusion of polymers caused by the extrusion of polymeric materials at shear stresses above a critical level. As the extrusion rate increases, the internal strain on the polymer also increases until the critical shear rate is reached. Typically, melt-fracture limits the rate and extent a polymer can be meltdrawn and results in a distortion of the surface of an extrudate.
Processing temperatures during extrusion of polymers also limits the rate of production and may cause melt-fracture. As processing temperatures increase, the polymer flows more easily, thereby reducing the shear stresses and reducing the likelihood of melt-fracture.
The geometry of spinneret orifices also may produce melt-fracture and thereby affect both extrudate appearance and production efficiency. The smaller the spinneret orifices, the higher the increase in pressure on the polymer as it passes through the orifices. This also increases internal strain on the polymer until a critical shear level is reached and the extrudate becomes rough. See D. Chang, Rheology in Polymer Processing, pps. 304-315 (Academic Press, Inc., New York, N.Y. 1976).
The occurrence of melt-fracture has been ordinarily regarded a problem which must be avoided. Because it is viewed as a problem, there have been attempts to manipulate processing conditions in the production of fibers to prevent the occurrence of melt-fracture. For example, processing conditions such as temperatures have been manipulated. Others have attempted to prevent melt-fracture by using a die structure configuration to alleviate pressure forces on the polymer as it is extruded.
There have been instances where melt-fracture has been purposefully induced for particular reasons. For example, U.S. Pat. No. 3,574,808 to Matthews et al. teaches a method of forming patterned articles such as containers, film, and sheetstock by blow molding, free blowing, and casting processes employing melt-fracture. U.S. Pat. No. 3,415,796 to Souder et al. teaches a method of forming a film, such as films used as sandwich or trash bags, with a roughened surface, and U.S. Pat. No. 4,615,858 to Su teaches a method for forming polymeric products with decorative patterns resulting from melt-fracture. None of these patents, however, teach the use of melt-fracture in the production of fibers, or suggest that melt-fracture can serve any beneficial purpose in the production of melt extruded fibers or nonwoven fabrics.